Flunk said:
The last boss in Gears of War, the entire game is fairly easy and that last fight is frustratingly hard, make one mistake and start over, and over, and over. Repeat until annoyed.
JEEZUS H. I can remember a friend and me were co-opping RAAM on insane, no joke it took upwards of 40 attempts to beat him, it was the damn krill that did it nearly every time.
For me though, worst bosses have got to be:
Dead Rising: that ridiculous fight on top of a tank with that army dude who blocks everything you try to hit him with, and when he hits you, you go FLYING into the horde of zombies surrounding the tank
That kreiger guy near the end of farcry. He was invisible part of the time, could jump around the room dodging your shots, was super accurate with his high powered gun, killed you in one hit with melee and shot fucking grenades out of his chest. I never got past that dude. I did play it on hard first time through, mind.